The present invention relates to a galvanic isolation coupling of a current loop comprising an operational amplifier as a part of the current loop and an optoisolator comprising two receivers.
Current loops are commonly used for conveying measuring information. A constant-current signal passing through the current loop is generated by a measuring sensor and a measuring transmitter, and a variable to be measured can be e.g. temperature or pressure. The constant-current signal has a typical magnitude of 4 . . . 20 mA, whereby the lower limit of the measuring range of the variable to be measured is set for a 4 mA current signal, and correspondingly, the upper limit of the measuring range is set for a 20 mA current signal.
It is often desirable that the current loop which carries the current signal is galvanically isolated from the circuit utilizing measuring information. Measuring information is utilized as control equipment feedback, for instance. Galvanic isolation allows the measuring information to be processed in potential which differs from the current loop, whereby the reliability of the processing improves and the structure of the required couplings is simplified.
In order to get the information of the current signal in the current loop transferred undistorted to an isolated circuit, the isolation coupling should be highly reliable in structure and operation. Distortions occurring during the isolation have been a drawback with prior art isolation couplings of current loops, and as a consequence it has been difficult to utilize the measuring signal in an appropriate manner.